More generally, the invention relates to the field of electric motors having permanent magnets such as brushless, electronically commutated DC motors and other permanent magnet motors, and in particular those configured as inner rotor motors. In general, inner rotor motors consist of a rotor arrangement which is mounted onto the motor shaft and includes one or more permanent magnets, as well as a stator arrangement, such as a stator core, which is built up of metal laminations that carry windings. The rotor arrangement is coaxially inserted into the stator arrangement. For outer rotor motors, the rotor arrangement encloses the stator.
FIG. 11 shows the basic construction of an electric motor having a housing 114 in which a stator arrangement 118, a rotor arrangement 116 and bearings 126, 128 are accommodated to rotatably support the rotor arrangement. The stator arrangement 118 includes stacked metal laminations 155 and windings 160 and encloses an inner space into which the rotor arrangement 116 can be inserted. The rotor arrangement 116 includes the shaft 110, a back iron yoke 112 and permanent magnets 122. The bearings 126, 128 supporting the rotor arrangement can be integrated into a flange 124 in the motor housing 114.
FIG. 11 serves to explain the basic construction of an electric motor, with the rotor of the invention having an essentially cylindrical rotor core in which the permanent magnets are embedded.
According to the prior art, rotors with embedded magnets are generally known. A rotor configuration having a multi-polar design resembling a spoked wheel with radially extending embedded magnets is revealed, for example, in “Design of Brushless Permanent-Magnet Motors”, J. R. Hendershot Jr. and T J E Miller, Magna Physics Publishing and Clarendon Press, Oxford, 1994. As shown in this publication, it is known to manufacture a rotor with embedded radially extending magnets that are protected by means of a ring or a tube surrounding the rotor. The rotor in which the magnets are embedded is used as a back yoke.
A conventional form of rotors with embedded magnets is also revealed in EP 0 691 727 A1. This publication shows a number of permanent magnets which are inserted into slots formed in the rotor allowing the permanent magnets to be inserted into the rotor from the outside. At their radially inner ends, the permanent magnets are enclosed by the material of the rotor core.
Rotors with embedded permanent magnets have the basic advantage that the magnets can be fully encapsulated so that the rotor can also come into contact with aggressive media without the magnet material needing special surface protection to prevent corrosion etc. However, the described rotor design has the disadvantage that stray flux is generated by the rotor core in the vicinity of the shaft.
To prevent such stray flux from arising, it has been suggested in the prior art to place a sleeve made of magnetically non-conductive or low-conductive material onto the shaft onto which the flux guide elements of the rotor core are then fixed, between which the permanent magnets in turn are embedded. Such a design is revealed, for example, in EP 0641 059 A1; EP 0 803 962 A1; and DE 101 00 718 A1. Although this construction represents a great improvement on the prior art as described above in terms of the magnetic circuit and the distribution of magnetic flux density in the rotor, it is costly to manufacture and, due to the many individual parts, problems in the mechanical construction, such as an addition of tolerances, could arise.
EP 0803 962 A1 additionally shows that the slots to accommodate the permanent magnets have a bridge on their outer periphery to fully protect the permanent magnets from the outside.
WO 00/57537 describes a multi-polar permanent magnet rotor for an electric motor having embedded magnets which are disposed in such a way that a concentration of flux is produced. The permanent magnets are formed as flat cubes which are disposed like spokes radially to the rotor axis in recesses that are arranged between the flux guide elements which are fixed to the rotor. In assembling the magnets and the flux guide elements, the permanent magnets are formed as adjacent half-elements representing one pole respectively, and both the permanent magnets and the flux guide elements are attached to the shaft via a sleeve.
U.S. Pat. No. 4,327,302 describes a rotor construction having embedded permanent magnets for use in an electric motor. The rotor is made of stacked punched-out metal laminations which have V-shaped openings, with a permanent magnet being embedded in each of the two legs of each slot so that the permanent magnets extend essentially like spokes through the rotor core. At the apex of the opening, two adjacent permanent magnets are bridged by a free space which is filled with air or a non-magnetic material. The purpose of the above arrangement is to provide a simple, compact construction for an electric motor with high output power.
EP 1 309 066 describes a rotor for an electric motor which is constructed in essentially the same way as described above in reference to U.S. Pat. No. 4,327,302. With this construction, EP 1 309 066 aims to keep stray flux via the shaft as low as possible while providing a simple design for the lamination stack. Another method of constructing a rotor having embedded magnets is shown in EP 0 872 944 A1. The magnets are arranged in a radial direction, or parallel to a radial direction, to the rotor. In EP 0 872 944, the permanent magnets are disposed in a so-called double-spoke configuration. Each of these “double magnets” consists of a pair of permanent magnets whose direction of magnetization is substantially the same. They can be arranged parallel to each other as in the cited publication or inclined at an angle to each other. This arrangement goes to improve the running performance of the electric motor and, in particular, to reduce cogging torque and torque ripple.
Other published patents in respect of rotors with embedded magnets include GB 1,177,247; EP 0 955 714 A2; and U.S. 2002/0067096 A1.
The rotor presented in the invention preferably finds application in a brushless DC motor or another permanent magnet synchronous motor. Such motors can be used in a great variety of applications, including spindle motors for disc drives, motor-assisted systems in motor vehicles such as steering and braking systems, electric tools and many other applications.
The radial arrangement of the permanent magnets embedded in the rotor core gives rise to the problem of stray flux in the region of the shaft onto which the rotor is mounted. The shaft is usually made from steel and acts as an extra back yoke for the magnetic flux through the rotor core. This gives rise to considerable magnetic stray. This problem can be countered by fitting a sleeve made from a magnetically non-conductive or low-conductive material to the shaft to which the flux guide elements of the rotor core are fixed, between which in turn the permanent magnets are embedded. This construction method is relatively costly and requires extra individual parts.
In EP 1 309 066 cited above, to reduce stray flux via the shaft, it is suggested to construct the rotor as a lamination stack in such a way that the annular lamination has U-shaped punched out sections distributed evenly in a circle. The legs of the punched out sections extend radially outwards and a permanent magnet is inserted into each leg of the U-shaped punch-out. An air gap is formed between the radially inner ends of the permanent magnets within the punchouts which reduces stray flux to the shaft. Tests made by the applicant found that a non-negligible amount of stray flux still arises in this construction.
The object of the present invention is to submit a rotor for an electric motor which has embedded magnets and is simple to manufacture but nonetheless prevents the above problem of stray flux being generated in the region of the shaft.